1. Field of the Invention
The present invention relates to a technique for measuring optical defects of glazings and particularly motor vehicle glazings.
2. Description of the Related Art
Industry increasingly seeks to control the quality of the products that it produces. This is true in particular of the optical quality of glazings. To achieve this object, all the production parameters are kept in very narrow ranges. Nevertheless, a mishap is always possible, and, even if it is detected, its effect on the product quality is not always known so that an inspection of the quality of 100% of the production is essential in many cases.
For glazings, there is often a need to continuously evaluate their optical quality. It may be desired, in particular, to select the glazing leaving the production lines for a particular use, such as, for example, a mirror intended for scientific applications or a thin flat glass intended to be transformed into a very inclined windshield. In general, moreover, the optical quality of the windshields of modern motor vehicles must be thoroughly controlled. This criterion touches on the safety of the motor vehicles. Further, the shapes of the windshields, their inclination, the materials from which they are produced--very thin glasses or even transparent polymers--require a very elaborate inspection of the optical quality, often for 100% of the product.
The existing inspection methods mainly use projection techniques, either with a localized beam such as, for example, that of a laser, or by projecting a test pattern through a zone of the windshield. A technique using a localized beam, such as that of U.S. Pat. Nos. 4,398,822 or 4,453,827, which relate to aircraft windshields, makes possible a precise measurement, in particular of the angular deviation undergone by the light rays in going through the concerned location, but it requires very long measuring periods if it is desired to have an overall evaluation of the windshields. And this is the case when it is desired to be certain that a defect limit value is not reached by any of the motor vehicle windshields accepted in the output inspection of a production line.
The other known methods are global methods. That of U.S. Pat. No. 4,299,482, which performs a Fourier transform of the image of a striated screen as it is seen through the windshield, requires the making of a photographic printing plate and is thereby unsuited to a continuous inspection during production. The method described in German patent application DE 36 00 199 uses a moire generated between a striated screen and the projection through the windshield of an identical test pattern that is made to rotate slightly relative to the screen to obtain, in the absence of defects, rectilinear moire fringes. The deformation of the fringes is observed by eye and the measurement of the greatest deformation gives information about the greatest optical defect of the concerned field. But the display method would be very difficult to automate.
U.S. Pat. No. 4,647,197, on the other hand, proposes automating a conventional display method. Regular rectilinear scratches carried by a screen are observed through the windshield with a special camera. The scanning that is performed with a given speed makes it possible to "measure" the width of the dark or light lines in the direction of the scanning and to deduce from it data on the corresponding defect. This technique, which is based on a well-known standardized method, cannot provide better results. Moreover, the automation has limits which restrict the possibilities of the display method even more. In the standardized method, a global observation by eye makes it possible to locate the zone of the largest defect quickly, a measurement will then be made at the exact location where the width of the striped lines is at its optimum. The automatic measurement, on the other hand, performs a scanning with a defined span (it is 10 mm in the plane of the windshield in U.S. Pat. No. 4,647,197) and the optical effect is measured only at these specific locations: the risk is therefore great of not detecting the maximum defect. Further, the limits of a method which determines the thickness of striped lines in a single direction are well known; if the greatest defect does not have its dominant effect perpendicular to the lines, its value will be underestimated.